Rare fossil forces rethinking of early dinosaur evolution

A rare primitive theropod, or a bipedal, primarily carnivorous dinosaur, is bringing clarity to the early evolution of the group that includes more recent relatives like T. rex and birds. Tawa hallae, uncovered in New Mexican sediments from the Upper Triassic, has evidence of an air sack system surrounding the neck and braincase found in birds today, making this characteristic a much more primitive trait than previously thought. But even more enlightening is that a comparison of T. hallae with other early theropods finds that there is a curious mix of early North and South American forms at the base of the carnivorous dinosaur tree. The new research, published in Science, redefines the early evolution of this group as waves of migration from the south rather than as separate and endemic fauna. "We would expect that all of the theropod dinosaurs found in the quarry were related to each other," says Sterling Nesbitt, until recently a graduate student at the American Museum of Natural History who is currently at University of Texas at Austin. "But they are not. T. hallae and two other carnivorous dinosaurs from North America each have their closest relatives in South America."

During the Triassic (about 251 to 199 million years ago), while the supercontinent Pangaea was breaking into northern and southern protocontinents, dinosaurs were rare. Primitive dinosaurs did not dominate the terrestrial fauna yet and comprise only about 6% of tetrapod (four-limbed) fossils found at similar age localities. Instead, crocodylian relatives were common. But during this period, dinosaurs diversified into three distinct groups, the ornithischians, sauropods, and theropods.

T. hallae was discovered in 2004 when hikers stumbled across a few bits of bone at Ghost Ranch in New Mexico. An initial excavation by Alex Downs of the Ruth Hall Museum of Paleontology uncovered limbs and vertebrae that he showed to the other authors. They immediately knew that it was something new from North America's Late Triassic, according to Nesbitt. Nesbitt and colleagues began a full-scale excavation in 2006, part of which was captured during the filming of the IMAX Dinosaurs Alive! "Coincidently, the excavation of what became the holotype is in the film," says Nesbitt.

But more than the holotype (a specimen used to describe a species) was found at Ghost Ranch's Hayden Quarry. The team uncovered five to seven partially articulated individuals buried together in a relatively small pocket (about 2.5 meters by 2.5 meters) among a jumble of tens of thousands of other fossils. T. hallae was in very good condition, allowing a fairly complete reconstruction of a new species of carnivorous biped, and dated to about 215 million years old.

"Finding dinosaurs this old and this complete in an area that has been prospected for over a hundred years is surprising," says Mark Norell, Curator and Division Chair of Paleontology at the American Museum of Natural History. "This is near the site of the dinosaur 'graveyard' where early dinosaurs like Coelophysis have been found since 1947. Now we have more bones from what was early in dinosaur evolution."

In their current paper, the team not only describes T. hallae but fits it in the evolutionary tree with other known theropods. Looking at 15 other theropods and about 300 morphological characteristics, the least complicated, more parsimonious phylogenetic tree now places South American fossils Herrerasaurus and Eoraptor at the base. T. hallae and two other North American species, Chindesaurus and Ceolophysis, both have South American theropods as their closest relatives. This means that there were several waves of theropod migration from South America into North America. This movement correlates with the fossils from many other groups of animals, including crocodylomorphs, aetosaurs, and shuvosaurids, that are found on both land masses. However, another group of saurischian dinosaurs, the sauropods, have not been found in the North America during the Triassic.

"Our biogeographic analyses show that dispersal of early dinosaurs was prevalent during the Triassic, with the multiple theropods from the Hayden Quarry, clearly demonstrating this pattern," says Alan Turner, research associate at the Museum and an assistant professor at Stony Brook University. "We propose that early dinosaurs from South American got into North America at least three separate times. We don't know why sauropods were not in North America, but it looks increasing likely that sauropods were getting here but something was preventing them from staying."

T. hallae is also redefining the history of a very important feature found in many dinosaurs and today's birds, pneumatization of bone, or cavities filled with air. These air sacks often extend into the brain case and ear area. Because T. hallae has an indentation in the bone in the anterior tympanic part of the braincase, this species probably had air sacks filled with air near its ears. This means that this feature is much more primitive that previously assumed. T. hallae also shows evidence of pneumatization along the vertebral column, perhaps to lighten the skeleton.

"T. hallae shows us that some traits go further back in the evolutionary tree," says Norell. "The discovery of fossils of such primitive dinosaurs allows us to link the South American and North American theropod faunas for the first time. Now we can evaluate an entirely new set of biogeographic questions."